So-called private automatic radio telephone networks or trunked networks, where several user groups or even several user organisations share the same radio channels and other resources in a common network are widely used. Typical private radio network applications include energy production and distribution, communal and municipal services, such as transportation, construction and maintenance, and emergency services, such as police, fire alarm, security and ambulance. A private radio network may offer fast access within the user's own organisation, individual or group communication, data communication, etc. The system checks the subscriber authorization and allocates resources. An individual subscriber is unaware that he or she uses the same frequencies as other subscribers since the system automatically allocates a free radio channel for a call during the call set-up. Radio telephone systems of this kind usually have a rather limited geographical coverage, and so the user may easily get outside the service area of his or her own system, whereby he or she cannot use his radio telephone. Therefore, interest has been aroused in the possibility of using the same radio telephone in several different systems.
The specifications MPT 1327 and MPT 1343 issued by the British Department of Trade and Industry have become to be regarded as some kind of de facto standards in Europe for private radio telephone networks. MPT 1327 specifies signalling over the radio path, while MPT 1343 specifies the operation and structure of the radio telephone. Systems complying with these specifications have been and will be introduced in different European countries. Since radio telephones used in different systems actually comply with the same specification MPT 1343, they can, in principle, be transferred from one radio telephone network to another by varying the network-specific radio telephone parameters complying with the MPT 1343. The network parameters that are needed and can be stored in a radio telephone are defined on pages 6-1 to 6-8 of Part 6 of MPT 1343, particularly in Table 6-1.
According to the above Part of the specification, radio sections providing the user with a possibility of switching between the systems selected must have a sufficient parameter storage to meet the requirements of every system. According to the above specification, the majority of the network parameters must be stored in a memory of type A, i.e. read-only memory. The parameters varying during the operation of the radio section must be stored in a memory of type B or C, i.e. secured or unsecured memory, respectively, depending on whether the information may disappear when the radio telephone is turned off.
German Offenlegungsschrift 3,838,677 discloses a radio telephone in which the loss of important data caused by the user is prevented during the operation of the radio telephone by storing the data in a removable memory card, in which are also stored the identification data of the radio subscriber. These important data include, e.g., the last call and the mode of operation of the phone. The subscriber data can be transferred from one radio telephone of a system to another similar phone in the same system by means of a memory card, automatic control of individual subscriber data stored in the memory card preventing the use of the memory in a similar phone in another system.
German Offenlegungsschrift 3,721,360 discloses a radio device in which network-specific control values are stored in a removable memory to enable further development of the network and associated updating of the subscriber devices.